Simian virus 40 (SV40) can cause tumors when injected into susceptible hosts. We will isolate new SV40 mutants which are defective in the genes coding for two tumor antigens, t and T. These new mutants will be used to study the role of these putative oncogenic proteins in SV40 cell transformation and oncogenesis. Rat cells transformed by new mutants with genotypes t-Tts and ttsT- will be used to determine which transformed cell phenotypes are under the maintenance control of t or T. Immunocompatible rats will be innoculated with rat cells transformed by mutants with genotypes t-Tion and tionT- to determine whether t or T (or both) us involved in SV40 oncogenesis. Finally, these mutants will be used to study the mechanisms by which these putatative proteins may act to cause cell transformation and tumors.